Total joint replacement is an established treatment for severely impaired joints. Significant complications persist, however, which are directly influenced by fracture process occurring in the interfacial structures found in total joint replacement systems. The purpose of this research is to establish failure criteria for these structures using the principles of fracture mechanics. Two specific problems will be studied: surface damage occurring on the polyethylene surface of the tibial component in total knee replacement as a result of articulation with the mental femoral component and failure of the prosthesis-cement and cement-bone interfaces around the stemmed femoral component in total hip replacement. Both problems will examined in a similar manner. Failure criteria governing the occurrence of the problem will be experimentally established. Stress analysis will be performed to determine the stresses arising within the implant system which would lead to failure. These stresses will be examined as a function of the implant geometry. Facture analysis will then be performed to show that variations in component geometry will affect the propensity for failure. This analysis will employ the experimentally determined failure criteria. The goal is to demonstrate that design variables can be chosen to minimize the occurrence of such failure without adversely affecting other aspects of implant function. In this way, the long-term performance to total joint replacement will be improved.